ACPI / video: skip evaluating _DOD when it does not exist

commit e34fbbac upstream.

Some system supports hybrid graphics and its discrete VGA
does not have any connectors and therefore has no _DOD method.

Signed-off-by: Alex Hung <alex.hung@canonical.com>
Reviewed-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>